1. Field of the Invention
The present invention relates to a mobile communication cellular system, and, more particularly, to a technique of handoff control when a mobile station moves from one sector cell to other sector cell, each sector cell is provided by a radio base station using a CDMA (Code Division Multiple Access) transmission system for communication with mobile stations.
2. Description of the Related Art
FIG. 2 is a view showing an example of a structure of a conventional mobile communication cellular system using the CDMA transmission system.
As shown in FIG. 2, the mobile communication cellular system is constituted by a plurality of mobile stations 103-1 to 103-n, a radio base station 102 for transmitting/receiving radio signals with respect to communication with the mobile stations 103-1 to 103-n, and a radio base station control apparatus 101 for controlling the radio base station 102.
Further, a voice encoder 104 for encoding and decoding a voice signal is provided within the radio base station control apparatus 101, and TRXes (transmitter/receiver) 110-1 to 110-N for transmitting/receiving radio signals between the radio base station 102 and the mobile units 103-1 to 103-n are provided within the radio base station 102 in accordance with each of sector cells 1 to N thereof.
The handoff operation, which is carried out when a mobile station moves from one sector cell to other sector cell, in the above-described configuration will be described with referring to FIG. 3 as well as FIG. 2.
FIG. 3 shows a signal flowchart of the handoff operation in the mobile communication cellular system illustrated in FIG. 2.
In the mobile communication cellular system, a pilot indicating a ratio of Eb/I0 (Eb: energy per one bit, I0: interfering energy from another channel) is always transmitted from the radio base station 102 providing a plurality of the sector cells 1 to N to the mobile stations 130-1 to 130-n.
In each of the mobile stations 130-1 to 130-n, each strength of pilot (Eb/I0) transmitted from all sector cells are received, and compares the strength of pilot from the sector cell which is currently performing communication with the strength of pilot from each of adjacent sector cells for detecting a sector cell which is optimum for carrying out the communication, namely, a sector cell providing the strongest pilot is detected.
It is assumed that the mobile station 103-1 is currently communicating with the TRX 110-1 of the sector cell 1.
The strength of pilot from the sector cell 1 is compared with that of the strength of pilot from the sector cell 2 in the mobile station 103-1 and, if the strength of pilot from the sector cell 2 exceeds it from the sector cell 1, the message (Pilot Strength Measurement Massage) for requesting the handoff operation control is transmitted from the mobile station 103-1 to the radio base station 102.
The message sent to the radio base station 102 is further transferred to the radio base station control apparatus 101.
When the message for requesting the handoff operation control is transmitted, the radio base station control apparatus 101 analyzes which sector cell providing pilot whose strength exceeds the strength of pilot of the sector cell 1 with which the mobile station 103-1 is currently communicating. In this case, since the strength of pilot of the sector cell 2 exceeds the pilot of the sector cell 1, it is determined that the mobile station 103-1 is subjected to perform the handoff operation from the sector cell 1 to the sector cell 2.
Then, the radio base station control apparatus 102 transmits the handoff direction message to the mobile station 103-1 through the radio base station 102 for assigning a communication channel which can be used at the sector cell 2. At this time, the message is transferred to the mobile station 103-1 through the communication channel currently being communicating between the sector cell 1 and the mobile station 103-1. Also, at the same time, the communication channel activating message is transferred from the radio base station control apparatus 101 to the sector cell 2 for activating the communication channel of the sector cell 2 having been assigned to the mobile station 103-1 for the handoff operation
The communication channel of the sector cell 2 is activated in accordance with parameters set in the communication channel activating message, and a transmission power of radio signal for the communication channel at the sector cell 2 is set to the same transmission power of the sector cell 1 or a maximum transmission power.
When the handoff direction message is received by the mobile station 103-1, the communication operation using a newly assigned communication channel is tried. If the trial operation is succeeded, a handoff completion message is transmitted from the mobile station 103-1 to the TRX 110-2 of the sector cell 2 in the radio base station 102.
Here, if two or more communication channels are assigned in the handoff direction message, it means that the soft handoff operation which is the handoff operation without interruption of communication is instructed. In such a case, communication trials are performed for all assigned communication channels, and in the radio base station 102, the handoff completion messages are received in all communication channels for which the communication trial has been succeeded.
When a handoff completion message is received in the radio base station 102, information, in which a sector cell number which has received the message is added to the handoff completion message, is transmitted to the radio base station control apparatus 101. If two sector cells have received the handoff completion message, the two handoff completion messages each having a different sector cell number are transmitted to the radio base station control apparatus 101.
When the handoff completion message is received in the radio base station control apparatus 101, the switching control to connect a voice encoder 104 to a channel encoder (not shown) of the corresponding sector cell is performed in accordance with the sector cell number which has been added to the handoff completion message. If a plurality of handoff completion messages have been transmitted from two sector cells, the switching control to connect the voice encoder 104 to those channel encoders are performed.
However, in the conventional mobile communication cellular system, the above-mentioned flow control is performed each time whenever the handoff operation is carried out. Thus, in the case where the area of each sector cell is small, a number of times of the handoff operation increases and it causes problems such that it requires more control processing capabilities in the radio base station control apparatus than expected and also, it requires more radio signals for message transfer between radio base stations and mobile stations for the handoff operation than expected.